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81. It is difficult to perform electrostatic experiments on humid days, why?

As the dry air behaves as a insulator of electricity but increase in the moisture decreases the insulator behavior and makes it a good conductor. Hence, it is difficult to perform electrostatic experiments on humid days.

 

82. Why are sharp points strictly avoided in electrical machines?

We know that the surface density of a charge around sharp edges or points of a conductor is very great. Therefore, the strength of the electric field near the point is very great. The intense electric field breaks down the insulation of air, and sends a stream of charged molecules away from the point. In order to avoid the leakage of charge from the electrical machines, sharp points are strictly avoided in electrical machines.

 

83. A hollow sphere is charged by touching inside with a charged rod. If the inside of the sphere is connected to a neutral gold leaf electroscope, what will happen?

As the charges resides on the outer surface of the sphere and there are no charges inside the hollow sphere, hence there will be no divergence on the leaves of the gold in the gold leaf electroscope.

 

84. Is it safe to stay inside a building or outside the building to be safe from thunderstorm?

As the electric field is zero and potential is constant inside a hollow charged conductor. A hollow conductor is an electrostatic shield. The house also behaves as an electrostatic shield. This means that if we stay inside the building , no light discharge can pass through the building and we remain safe. Hence, it is safe to stay inside a building than outside the building to be safe from thunderstorm.

 

85. Why is it dangerous to stand under a tree during a thunderstorm?

During thunderstorm if the lightening occurs, we provide path for the lightening to pass through our body to the ground if we are standing under a tree which results in the flow of high voltage through our body which may be fatal also. So, it is dangerous to stand under a tree during a thunderstorm.

 

86. Why is the upper portion of a lightening rod always pointed?

 

As the surface density of a charge around sharp edges or points of a conductor is very great. Therefore, the strength of the electric field near the point is very great. The intense electric field breaks down the insulation of air, and sends a stream of charged molecules away from the point. When a negatively charged cloud passes over the building, it induces a positive charge on the pointed ends of lightening rod and an equal negative charge at the metal plate. Due to discharging action of sharp points, a positively charged electric wind is set up that neutralizes the negative charge on the cloud. Negative charge on the metal plate is completely neutralized in the earth. If lightening occurs then the conducting rod sends it directly to the earth and save our building from damaging. Hence, the upper portion of a lightening rod is always pointed.

 

87. Why is the leaves of an electroscope always diverge when a charged body is bought close to it?

When a charged body is bought close to an electroscope then the disc gets charged by induction with opposite polarity as that of the charged body. At same time, leaves of an electroscope is charged with same charge as that of the charged body. Both leaves is charged with like charges so thy repel each other & diverge. Hence, the leaves of an electroscope always diverge when a charged body is bought close to it.

 

88. When a polythene piece is rubbed with wool, it acquires a negative charge. Is there a transfer of mass from wool to polythene?

When a polythene piece is rubbed with wool, it acquire a negative charge. Here, a positively charged body i.e. wool has lost some electrons and hence electron possess mass so, there is loss of mass in wool. On the other hand, a negatively charged body i.e. a polythene piece has gained some electrons from wool and its mass increases slightly. As there is a transfer of electrons from wool to polythene & electrons possess mass, we can say that there is a transfer of mass from wool to polythene.

 

89. Electric shocks may be felt on touching a running motor car. Why?

As the car contain metallic tank which gets charged due to friction with air when it is running. As our body behaves as conductor and when touched touch a running car then the charge which was stored on the metallic tank directly flows from our body to the ground results in the getting shock. Hence, electric shocks may be felt on touching a running motor car.

 

90. How many electrons are present in one Coulomb?

As we know that one electronic charge 'e' contains a charge of  1.6 10-19 Coulombs. Using unitary method gives

           

Hence, 6.25 1018 electrons are present in one Coulomb.

 

91. A man in an insulated metal cage does not receive a shock when the cage is connected to a high voltage source. Explain.

The charge resides on the upper surface of the hollow conductor if it is charged. Hence, the charge can not penetrate the inner surface of the hollow conductor and the person inside insulated hollow metal cage does not receive shock when the cage is connected to a high voltage.

 

92. Is it possible to transfer all the charge from a conductor to another isolated conductor?

Yes, it is possible to transfer all the charge from a conductor to another isolated conductor. This can be possible by placing the charged conductor inside the hollow insulated conductor and connecting both by a wire. The whole charge will shift to the outer surface of the conductor.

 

93. Repulsion is the surer test to distinguish the nature of the charge. Justify this statement.

There is always attraction between the unlike charges and a charged body and an uncharged body. But, there is repulsion between like charges. Hence, it attraction occurs between unlike charges and a charged body and an uncharged body then there is cent percent for the repulsion of like charges. Hence, repulsion is the surer test to distinguish the nature of the charge.

 

94. What kind of charge is produced when a silk is rubbed with human body?

As the human body is the good conductor, if silk is rubbed with human body then negative charge is produced on it which passes through human body directly to the earth (ground). Hence, it is difficult to produce charge by rubbing with the human body.

 

95. When an iron plate is placed between two charges, what will be the electrostatic force of attraction between them?

The electrostatic force 'F' of attraction between two charges q1 & q2 separated 'r' distance apart is given by

                   

where, eo & er are permittivity of free space & relative permittivity of the medium. When an iron plate (having the relative permittivity infinity i.e. ) is placed between two charges, the electrostatic force of attraction between them becomes zero according to following relation.

                  

96. There is a surface that does not include any charge. What will be the value of electric flux and electric field?

There is a surface that does not include any charge i.e. q = 0. From Gauss theorem, the total flux f through a closed surface is equal to the ratio of the net charge 'q' enclosed by the surface to the permittivity of free space e0 i.e.

         

If q = 0, so from above equation electric flux also becomes zero i.e. no lines of force passing through that surface.

But electric field may not necessary to be zero for that surface because if a charge is present at a point close to that surface, the electric field will be produced in that surface.

 

97. If the electric potential be constant throughout a given space, how does the value of electric field intensity vary through the region?

The electric field intensity is negative potential gradient with respect to the distance i.e.

                         

where, 'E' is the electric field intensity and 'dV' is the change in potential with respect to distance 'dx'.

When the electric potential remains the same throughout a given space, there is no change in potential with respect to distance. Hence, electric field intensity will be zero. Thus, if the electric potential be constant then the electric field intensity will be zero.

 

98. The electric potential at a point is one volt. What do you mean by this?

The electric potential at a point is one volt. This means that if we place a charge of 1 Coulomb at that point, the charge will have a potential energy of 1 Joule i.e. potential energy per unit charge is 1 Volt. 

 

99. Is it possible to have point where potential is zero but electric intensity is not zero? Give one example.

Yes, it is possible to have a point where potential is zero but electric intensity is not zero at a mid-point between two equal & opposite charges.

100. Why gravitational forces are neglected when computing the force between the charged objects?

As the gravitational force is dependent on the product of the masses of the charged bodies as well as universal gravitational constant. All these quantities are very small so, gravitational force becomes very small. While regarding the electrostatic force between the charge bodies both charge and proportionality constant are considerably higher. Hence, electrostatic force between the charge body is extremely higher than gravitational force. So, gravitational forces are neglected when computing the force between the charged objects.


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